Rubber products today are made from natural rubber and synthetic rubber or blends thereof. Natural rubber (NR) differs from synthetic rubber. Natural rubber is made from the milk of the rubber tree. This rubber milk, called latex, is a colloidal dispersion in an aqueous medium. Only small percentage of the latex is used directly and the greatest part is processed into hard rubber. Among many different types of synthetic rubbers, the most common synthetic rubbers are SBR (styrene butadiene rubber), BR (butadiene rubber), EPDM (ethylene propylene diene rubber), IR (isoprene rubber), IIR (isoprene isobutylene rubber), NBR (acrylonitrile butadiene rubber), SIS (styrene isoprene styrene), SBS (styrene butadiene styrene) and CR (poly-2-chlorobutadiene). Examples of rubber products include, but are not limited to, conveyor belts, flat and vee-belts, tires, sole material, sheet material, and punching products.
Rubber products are frequently made up of several rubber layers each with the same or a different chemical composition. During this “build up”, the rubber layers must adhere to one another adequately in their pre-vulcanized state. For example, an assembled tire blank is required to hold together for a fairly long period prior to vulcanization. It is therefore important that the rubber mixtures used have an adequate “tack.” The property termed “tack” is defined as the force required to pull apart two pre-vulcanized rubber mixtures which have been pressed together under certain defined conditions. While natural rubber mixtures generally have good tackiness, mixtures of synthetic rubbers are much less tacky and, in extreme cases, possess no tackiness at all. Therefore, it has been common practice to add a tackifier to less tacky rubbers or rubber mixtures to increase their tack. In synthetic rubber products, synthetic rubber adhesive compositions are employed to improve tack and provide good cured adhesion.
Rubber compositions containing a tackifier are generally formulated in internal mixers or on sets of rollers from a natural or synthetic rubber (e.g. styrene-butadiene copolymers, polybutadiene) or mixtures thereof. Rubber compositions also typically contain additives known in the art such as fillers, processing agents and vulcanizing agents. After formulation, the rubber composition is then used to manufacture a desired rubber product. As mentioned above, the rubber composition must remain sufficiently tacky during the manufacturing process, even when the process is interrupted for fairly long periods, which is not unusual particularly when manufacturing involves processes at different locations or requires storage and/or transport of pre-finished goods.
Even though a number of different materials are currently used as tackifiers, there remains a need to develop tackifiers which provides rubber compositions with increased tack. A particular need exists in the tire industry because of the poor tack of synthetic rubber compositions, such as commercial SBR-based tire compositions.
This invention answers that need. Tackifying resins of the invention, modified hydrocarbylphenol-aldehyde resins, have improved tack performing as good as or better than current tackifiers. The invention also provides a process for preparing modified hydrocarbylphenol-aldehyde resins and an improved rubber composition containing such resin.